Influence of asymmetric adsorption on electronic states of molecule studied by scanning tunneling microscopy and spectroscopy

The electronic states of unique molecule (Py-Per-Py), composed of two pyrene (Py1 and Py2) units, were investigated by using scanning tunneling microscopy and spectroscopy (STM/STS) at the liquid/solid interface. Bias-polarity-dependence of STM images of the Py1 and Per moieties were observed. STS results revealed that the apparent energy gaps for the Py1 and the Py2 were appreciably different due to possible asymmetric adsorption configurations. The apparent energy gap for the Per moiety measured by STS was in agreement with the hybrid density functional theory (DFT) simulations. In a parallel study, another molecule (Py-e-Per-e-Py) was also studied by using STM/STS for further confirmation of the proposed mechanism.